As used herein, a "semiconductor device" is a silicon chip (die) containing circuit elements on a face thereof, and a "semiconductor device assembly" is a silicon chip contained within a package and connected (wired or bonded) to conductive leads which exit the package.
A common type of semiconductor device assembly has a plurality of pins exiting the bottom surface of the package body, and is termed a Pin Grid Array (PGA). The present invention is especially applicable to PGAs.
The complexity of modern semiconductor devices results in a high number of pins. Pin counts in excess of one hundred are not uncommon. For high pin count packages, the pins are necessarily very thin and closely spaced. Spacing on the order of 0.070-0.100 inch (center-to-center) is not uncommon. Evidently, these pins are delicate, and caution must be exercised in handling the package to avoid damaging the pins so that they can be properly aligned with corresponding holes in a printed wiring board (e.g.) or in a socket to which the packaged device is ultimately mounted. Common damage modes include: 1) physical distortion of the leads, or 2) removal of plating from the leads due to scraping (the leads are commonly gold-plated). Damage to the ceramic package body itself by the metallic boat is also a concern.
In the process of packaging a semiconductor device, it is typical that the semiconductor die is inserted into an opening in the package. The package already has pins exiting the package body, and lead fingers within the package body. The lead fingers ar bonded (or wired) to the pins. After the semiconductor device is connected to the lead fingers (such as by wire or tape-automated bonding), a lid is applied over the package opening to seal the device within the package. During these packaging steps, there are many opportunities for causing damage to the pins while handling and processing the package.
FIG. 1 shows a typical semiconductor device assembly 100, for which the present invention is especially pertinent. The assembly includes a square, flat ceramic package body 102 having a top surface 102a and a bottom surface 102b. An opening 104 extends into the top surface of the package. A plurality of pins 106 extend outward from the bottom surface 102b, and are connected 10 (not shown) to lead fingers (die bond sites) 108 which extend to within the opening. This type of package is referred to as a Pin Grid Array (PGA).
A semiconductor die 114 is inserted into the opening 104, is attached to a die attach pad (not shown), and is connected to the exposed ends (bonding sites) of the lead fingers 108 by any suitable technique (e.g., wire bonding or tape automated bonding).
In many cases, selected pins, for example one pin at each of the four corners of the package 102, are provided with expanded "stops" 106a spaced from the bottom 102b of the package body 102, so that the package body 102 will sit a prescribed distance above a printed wiring board to which the assembly 100 is mounted. For purposes of this discussion, pins 106 having stops 106a are termed "guide pins".
It should also be noted that a central area (not shown) on the bottom surface 102b of the package 102 may or may not be free of pins, in an area corresponding roughly to the opening 104. (In the case that an array of pins covers the entire bottom surface 102, the package is referred to as a "fully-populated" pin grid array. In the case that a central area is free of pins, the package is a "partially populated" array.)
The top surface 102a of the package is has a metallic ring 110 formed about the periphery of the opening 104. After the semiconductor device 114 is attached to the lead fingers 108, a lid 116 is secured over the opening 104, "capping" the package. The lid is essentially a flat metal (or ceramic) plate, and is evidently slightly larger than the opening 104. The lid is commonly sized to fit over the ring 110. A solder "preform" foil 118, of similar size and shape as the ring 110 is provided between the lid 116 and the ring 110, so that the lid may be secured to the package body 102 simply by heating the entire assembly, causing the preform 118 to seal and secure the lid 116 to the top surface of the package 102, over the opening 104.
As mentioned above, in the process of mounting the die in the package, and capping the package, the entire assembly is typically passed through an oven (i.e., to melt the preform 118, inter alia). Conveyor transports are common, and the packages may be placed on "boats" for the journey through the oven.
FIG. 1 shows a portion of boat 150, positioned to receive the semiconductor device assembly 100. The boat 150 is typically an elongated metal structure capable of supporting and transporting a number (e.g., six) of semiconductor device assemblies 100. The boat 150 is a rigid metal structure having a platform portion 120, a top surface 120a, a bottom surface 120b and an area designated by dashed line 122 for supporting the semiconductor device assembly 100. The boat is provided with side portions 124 extending downward from opposite side edges of the platform portion 120, so that the top surface 120a is maintained a suitable distance above a conveyor belt (not shown).
A number of holes 126 are provided through the platform portion 120, and the holes 126 are sized and spaced to allow the pins 106 to pass easily through the top surface 120a so that the package body 102 rests directly on the platform 120.
Typically, the holes 126 are quite (about 0.020 inches) larger than the diameter of the pins 106, to allow easy insertion of the pins and to prevent gouging or scraping of the pin plating. This evidently allows the package 102 to move, and allows the pins to contact the platform 120. Some of the holes 126 may be only a slightly (e.g., 0.005 inches) larger than the pins 106, and these particular holes "126a " act as guide holes, for accurately positioning the package.
Evidently, the holes 126 are capable of gouging the typically softer plating material on the pins 106, due to burrs and the like. This problem is exacerbated by the smaller guide holes 126a. Further, there are expansion problems when the assembly 100 is transported through an oven on the boat 150, in that the plate portion 120 can contact the pins 106.
In cases where stops 106a are not provided, the package body 102 rests directly atop the platform portion, which can cause damage to brazing 106b (see FIG. 3) at the junction of the pin 106 and the package body 102.
Further, the boat is typically formed of a material (e.g., stainless steel) galvanically dissimilar from the pin material (e.g., gold-plated copper) which, when the assembly is transported through an oven can cause undesirable corrosion, electrolysis, or the like, of the pins 106.
As shown in FIG. 1, a large cutout 130 extends through the plate portion 120 in a region essentially central to the area 122. This cutout 130 allows automated machinery (not shown) to lift the finished semiconductor device assembly 100 up off of the boat 150 both during and at the completion of fabrication (packaging). The package is generally lifted once per operation (e.g., die attach, wire bonding, etc.).
As shown in FIG. 1, additional cutouts 132 may be provided within the periphery of the area 122, to minimize the number of holes 126. Only relatively few holes 126, compared with the total number of leads 106, are needed.
Taking into account the cutout 130 and additional cutouts 132, the plate portion 120 is left with "bridges" 134 at two opposite sides of the area 122, and the bridges have a sufficient number of holes 126 to properly align and maintain reasonably motionless a semiconductor device assembly 100.
In the process of inserting the leads 106 of the assembly 100 into the holes 126, because the boat is the direct support for the semiconductor device assembly, contact between the leads and the boat occurs. During the installation and removal of the package from the boat, physical bending of the pins may occur because of the close tolerances necessary to maintain package alignment. At the points of contact, motion of the semiconductor device assembly on the boat has been observed to cause the pin plating to be worn or scratched away, exposing either the under coat or the base metal.
What is needed is a boat assembly that does not damage pins or package bodies.